The present invention relates to static excitation systems for large fluid cooled dynamoelectric machines, or electric generators, and, more particularly, to static excitation systems using liquid cooled components, especially a liquid cooled excitation transformer.
Excitation systems for large dynamoelectric machines, such as turbine-generator combinations employed by electric utilities, have grown in power rating along with power ratings of generators themselves. (As used herein, the term "large dynamoelectric machine" refers to one which has a power rating in excess of approximately 50 megawatts.) Early excitation systems included rotating power sources, such as a separate DC generator driven by the turbine-generator shaft, that supplied excitation current to rotating field windings through slip rings and brushes. Another approach employed an AC exciter driven by the turbine-generator with rectification and control of the excitation voltage available in external stationary rectifier banks. Still another approach used diode rectifiers mounted on the rotor, wherein the entire rectification power source rotated, and control was effected through electromagnetic flux linkages with the rotating components. An example of an excitation system having rotating rectification means is found in U.S. Pat. No. 3,768,002--Drexler et al, issued Oct. 23, 1973, and assigned to the present assignee.
Excitation systems having less severe cooling demands are possible, particularly where a high response ratio is not required. A simple excitation system, for example, appears to be disclosed in U.S. Pat. No. 3,132,296 issued May 5, 1964 to Nippes. However, the excitation system in the U.S. Pat. No. 3,132,296 is solely directed toward utilizing the third and higher harmonics of the fundamental frequency of the rotor magnetic flux. Such systems as disclosed therein are not practical for large dynamoelectric machines. The third or other higher harmonic is not capable of providing sufficient power to the excitation system of a large dynamoelectric machine for producing the required high level of generator power output.
A separate category of excitation systems is referred to as "static" because the excitation power source does not rotate, but is stationary or static. A compound excitation system of the static type is shown, for example, in U.S. Pat. No. 3,702,965--Drexler et al. and U.S. Pat. No. 3,702,964 --Kudlacik et al, both issued Nov. 14, 1972, and both assigned to the present assignee. The U.S. Pat. No. 3,702,964 and 3,702,965 describe a static excitation system for a large dynamoelectric machine wherein the excitation transformer may be disposed in a sealed enclosure that is separate from, yet physically closely adjacent to, the generator casing. It is clear from the description of the cooling system for the excitation transformer and the respective references to U.S. Pat. No. 2,695,368--Kilbourne, issued Nov. 23, 1954, and assigned to the present assignee, that the coils of the excitation transformer of the '964 and '965 patents are cooled by internal liquid coolant available from the generator, whereas the core laminations and main body of the excitation transformer are cooled by the same gas, typically hydrogen, which is used to cool the generator.
An excitation transformer using a winding and core coolant system that is isolated from the generator cooling system could be readily disconnected from the generator and conveniently repaired and/or replaced while an auxiliary excitation source is connected to the generator, thus reducing expected outage, or downtime, for the generator. In addition, a transformer sharing the gas cooling supply of the generator is generally situated either in a cooling dome of the generator casing, or housing, or in a pressurized, gas tight enclosure separate from the generator casing. In either case, the cooling dome or pressurized enclosure must be fabricated to the same specifications, such as the boiler code, as is the generator housing itself, in order to withstand internal forces without causing external damage. It would be desirable to provide a liquid-cooled excitation transformer since, by cooling the entire transformer, including windings, core and electrical connections with a liquid, the transformer can be disposed outside the pressurized gas environment and much of the expense for materials, processes and time required to fabricate the containment structure of the transformer can be eliminated, resulting in an overall saving for the ultimate customer. In addition, replacing a gas coolant with a liquid coolant would generally permit a smaller transformer for the same equivalent rating to be fabricated, or, if desired, more core laminations could be added to the same size package, thereby increasing the rating of the transformer, and ultimately the available generator output power.
It may be possible to provide coolant isolation between the excitation transformer and the generator by disposing the excitation transformer relatively remote from the generator and using separate coolant sources. Such orientation is typically employed when an oil-filled transformer, having an oil such as Pyranol for internal cooling is used. It is believed that these oil-filled transformers are generally not disposed within the same room as the generator, in part due to potential detrimental environmental effects to operating personnel. However, in order to minimize the length of coolant connections required, and further to minimize the length of electrical conductors from the generator to be coupled to the primary windings of the excitation transformer and length of electrical conductors from the secondary windings of the excitation transformer to be coupled to rectifier means and ultimately to field windings of the generator, it is desirable that the excitation transformer be disposed closely proximate the generator, and, certainly, within the same room or compartment of a building as the generator.
In the static excitation system described in U.S. Pat. No. 4,477,767--Cotzas, issued Oct. 16, 1984, and assigned to the present assignee, the excitation system is described as being disposed preferably within the generator housing in a cooling dome, directly in the flow path for and cooled by the same coolant fluid that is used for cooling the main portion of the generator. Although the electrical configuration of the excitation circuit of the U.S. Pat. No. 4,477,767 permits use of a smaller excitation transformer over previous transformers, it is still desirable further to reduce the size of the excitation transformer and to isolate coolant flow within the excitation transformer from the primary generator coolant flow.
In order yet further to reduce the size of the excitation transformer, it would be desirable that the transformer be cooled with a liquid, such as water, having a higher thermal conductivity than gases presently used for cooling. When water is used directly to cool the core of a transformer, it is generally necessary that the water be deionized to reduce its electrical conductivity. It would be preferable to use an existing source of deionized water, rather than provide a separate facility for deionizing water, since initial capital cost for establishing the separate facility may be expected to weigh heavily against benefits to be obtained by use of the present invention, when considered by anticipated sophisticated users, like commercial power utilities, in any financial or cost/benefit analysis for determining feasibility of installing an excitation system and liquid cooled transformer in accordance with the present invention.
Accordingly, it is an object of the present invention to provide a smaller excitation transformer for a static excitation system of a large dynamoelectric machine than those transformers previously used.
A further object of the present invention is to provide an excitation transformer which may be disposed closely proximate the dynamoelectric machine.
Another object of the present invention is to provide a static excitation system wherein coolant flow within components of the excitation system is isolated from the coolant system of the large dynamoelectric machine.
Still another object of the present invention is to provide a static excitation system wherein a liquid coolant is used to cool the core and windings of a transformer of the excitation system.
Yet another object of the present invention is to use an existing source of deionized water for providing one type of liquid coolant to the excitation transformer.
A further object of the present invention is to increase the output rating of an excitation transformer having the same outline dimensions as a previously used excitation transformer.
Other objects of the present invention are to eliminate undesirable features of a pressure vessel enclosure that is required when using a gas cooled transformer and to improve access to the transformer for inspection and maintenance.